Mode-selective spurious suppression in wideband LLSAW filters with oppositely oriented LiNbO3 bilayer on SiC

This work proposes and experimentally demonstrates an oppositely oriented lithium niobate (LN) bilayer on SiC platform for high-frequency wideband longitudinal leaky surface acoustic wave (LLSAW) filters targeting Wi-Fi 6 bands with 5G New Radio coexistence. Through finite element method simulations and mutual-energy analysis, we clarify a bilayer-LN design paradigm for mode-selective excitation in heterostructure SAW devices, enabling destructive cancelation of the low-frequency shear horizontal surface acoustic wave (SH-SAW) mutual energy while preserving efficient LLSAW excitation. Experimentally, resonators fabricated on the optimized +X-LN/−X-LN/SiC bilayer exhibit strongly suppressed low-frequency SH-SAW spurious responses over wavelengths from 1.1 to 1.5 μm, while maintaining LLSAW electromechanical coupling (kt2) up to 20.8% and Bode-Q values above 550 near 5.5 GHz. A corresponding ladder filter centered at 5.44 GHz further achieves a minimum insertion loss of 1.91 dB and a large 3-dB fractional bandwidth of 13.9%, with the parasitic transmission near 4 GHz reduced from 19.9 to 4.3 dB compared with the single-layer LN/SiC reference. These results establish oppositely oriented LN bilayer on SiC as a viable prototype for enabling spurious-free wideband LLSAW filters, as well as a scalable interlayer coupling design paradigm for mode-selective transduction in heterogeneous SAW devices.